In chemical plants such as a methanol plant and an ammonia plant (a urea plant is included), high-temperature and high-pressure steam is used. FIG. 1 shows an example of a steam system for controlling the steam.
The steam system 2 has a high-pressure header 4 which stores high-pressure steam therein and a low-pressure header 6 which stores low-pressure steam whose pressure is lower than that of the high-pressure steam. In some plants, a header corresponding to the low-pressure header 6 in FIG. 1 may be referred to as a medium-pressure header.
The high-pressure header 4 is connected to a waste-heat boiler 8. The waste-heat boiler 8 supplies high-pressure steam to the high-pressure header 4. A supply system of the waste-heat boiler 8 has a safety valve 10 and a discharge valve 12. When a steam pressure of the supply system exceeds a first predetermined pressure, a controller of a discharge valve 12 gradually increases valve opening set to be fully closed in normal time to release steam to the outside of the system. When pressure of the supply system exceeds a second predetermined pressure set larger than the first predetermined pressure, the safety valve 10 is opened depending on the steam pressure to release steam to the outside of the system. The high-pressure header 4 is further connected to an auxiliary boiler 14. The auxiliary boiler 14 supplies high-pressure steam generated by the auxiliary boiler (package boiler) to the high-pressure header 4. Pressure of the steam supplied by the waste-heat boiler 8 is higher than pressure of the steam supplied by the auxiliary boiler 14.
The low-pressure header 6 has a discharge valve 30. When steam pressure in the low-pressure header 6 exceeds a predetermined discharge valve control start pressure, a controller 32 of the discharge valve 30 gradually increases the valve opening set to be fully closed in normal time to release steam to the outside of the system. This control is performed by means of a PI controller using a difference between a measurement value PV of the steam pressure in the low-pressure header 6 and a discharge valve MV set to be slightly larger than a target value of the steam pressure in the low-pressure header in normal time. A pressure set value of the controller 32 of the discharge valve 30 is larger than a pressure set value of a low-pressure side controller 27 of a turbine bypass valve 23 described later.
The low-pressure header 6 further has a safety valve 28. When a steam pressure exceeds a safety valve control start pressure set to be larger than the discharge valve control start pressure, the safety valve 28 is opened depending on the steam pressure to release steam to the outside of the system. The low-pressure header 6 further supplies low-pressure steam to another process 34.
The high-pressure header 4 is connected to a turbine 16. High-pressure steam in the high-pressure header 4 is introduced into the turbine 16 through a turbine inlet piping 18. The turbine 16 is driven by the high-pressure steam, supplies mechanical energy to external apparatuses not shown and discharges steam with lower pressure. A part of the discharged steam is supplied to the low-pressure header 6 through a turbine outlet piping 20. Another part of the steam is supplied to a condenser not shown and the like.
The steam system 2 further has a turbine bypass line 22 connecting the high-pressure header 4 to the low-pressure header 6. The turbine bypass line 22 has the turbine bypass valve 23 for controlling a flow of steam flowing therein. When the turbine bypass valve 23 is opened, high-pressure steam in the high-pressure header 4 bypasses the turbine 16 and is supplied to the low-pressure header 6 through the turbine bypass line 22.
The turbine bypass valve 23 is controlled by operating a solenoid according to a control signal sent from a control part 24. The control part 24 has a high-pressure side controller 25, a low-pressure side controller 27 and a higher-order selector 26.
The high-pressure side controller 25 receives an input of a high-pressure side pressure being a plant value obtained by measuring pressure of the high-pressure steam in the high-pressure header 4. Based on a pre-stored process, the high-pressure side controller 25 generates high-pressure side MV for instructing opening of the turbine bypass valve 23 from the input high-pressure side pressure and outputs the high-pressure side MV. The high-pressure side MV is generated, for example, according to a PI control based on the difference between the high-pressure side pressure and the high-pressure side pressure set value. The pressure set value of the high-pressure side controller 25 is smaller than the pressure of the steam supplied by the waste-heat boiler 8 and is larger than the pressure of the steam supplied by an auxiliary boiler 13.
The low-pressure side controller 27 receives an input of low-pressure side pressure being a plant value obtained by measuring pressure of the low-pressure steam in the low-pressure header 6. Based on a pre-stored process, the low-pressure side controller 27 generates the low-pressure side MV for instructing the opening of the turbine bypass valve 23 from the input low-pressure side pressure. The low-pressure side MV is generated, for example, according to a PI control based on the difference between the low-pressure side pressure and the low-pressure side pressure set value.
The higher-order selector 26 receives inputs of the high-pressure side MV and the low-pressure side MV, selects the larger value of them as MV for control, controls the turbine bypass valve 23 and sends steam of controlled amount from the high-pressure header 4 to the low-pressure header 6. According to such control, when steam pressure in the high-pressure header 4 becomes higher than a predetermined level, the steam pressure in the high-pressure header 4 can be decreased. Furthermore, when steam pressure in the low-pressure header 6 becomes lower than a predetermined level, the steam pressure in the low-pressure header 6 can be increased.
The low-pressure header 6 is further connected to a low-pressure steam supply system not shown. The low-pressure steam supply system supplies low-pressure steam to the low-pressure header 6. The low-pressure steam supply system is controlled by a control device which previously stores a low-pressure side flow control SV therein. When pressure in the low-pressure header 6 exceeds the low-pressure side flow control SV, the amount of steam supplied from the low-pressure steam supply system to the low-pressure header 6 is decreased.
Japanese Laid-Open Patent Application JP-A-Heisei, 11-257018 describes an invention on a steam turbine steam bypass device for smoothly releasing steam used on a turbine side to a high-pressure steam condenser when a steam turbine is shut down in an emergency due to break-down (at trip).
Japanese Laid-Open Patent Application JP-A-Heisei, 7-229405 describes a turbine bypass control method in a combined plant including: a turbine bypass having a turbine bypass valve connected to an inlet of a steam turbine; and a turbine governor for controlling the turbine bypass valve, wherein, when the turbine governor stops an automatic control of the turbine bypass valve, the turbine governor controls the turbine bypass valve using a pressure which is higher than the steam pressure at this time by a predetermined value as a setting pressure.